Field of the Invention
This invention relates to sheet metal forming processes, and more particularly, to a sensor system and sensing element for detecting draw-in of a metal blank during metal forming operations.
Sheet metal forming is one of the predominant processes in the manufacture of automobiles (about 300 parts per vehicle), and also is widely employed in the manufacture of aircraft, appliances, beverage cans and many other products. The popularity of this type of metal-forming/stamping is mainly due to the high degree of design flexibility and its low cost, both in the stamping and in subsequent assembly. It is especially attractive for the manufacture of high strength, low-weight components.
In a typical sheet metal forming process of the prior art, as shown in FIG. 1, a blank sheet 12 is placed between a blank holder 14 and a die 16. During the forming process, the sheet 12 under the blank holder 14 is drawn into the deformation zone 18 by a punch 20. The draw-in amount xe2x80x9cdxe2x80x9d is the translation of the edge(s) of the sheet 12 from an initial positions(s) 22 to the draw-in position 24. The success of a forming process depends entirely on how one designs the tooling geometry and forming parameters for a specific sheet material. The binder acts to provide a tangential restraining force to the sheet to determine how much material flows into the deformation cavity, the draw-in amount at each cross section. This restraining force is generated either through the frictional force between the flat binder and the sheet material or through a combination of the frictional force and the bending/unbending effect of material going through a drawbead 26 (see FIG. 1).
Excessive material draw-in leads to the occurrence of wrinkling, which is usually undesired in final sheet metal parts for functional reasons. Wrinkling is also unacceptable for aesthetic reasons in the outer skin panels where the final part appearance is crucial. Wrinkling on mating surfaces can adversely affect subsequent part assembly use and function, such as sealing and welding. In addition, severe wrinkles may damage or even destroy dies. On the other hand, an unduly limited material draw-in amount leads to tearing failure in the sheet metal, a result which is just as undesirable. Therefore, proper material flow and draw-in amount are critical to the ultimate success of a metal-forming process.
Another concern of the prior art is that draw-in amounts around the periphery of a sheet metal piece or product are usually uneven due to a combination of complex piece geometry and material anisotropy. An existing method to monitor a forming process uses punch forces at four tooling corners, but is not sophisticated enough for use with formation of irregular shapes. Local sensors currently used in such applications include a mechanically based linear transducer (LVDT type) for sensing the material draw-in, a mouse-like draw-in sensor and a local friction force sensor. The installation of such sensors requires either additional setup time with each forming or intricate tooling modification. The lack of effective and affordable local measurement tools has hampered the implementation of active control systems for industrial metal-forming processes, a state which in turn has resulted in long development time and inconsistent final sheet metal products.
In light of the foregoing, it is an object of the present invention to provide a sensor device and/or element, together with methods for its production and/or assembly, thereby overcoming various deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all its respects, to every aspect of this invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of this invention.
It can also be an object of the present invention to provide a sensor device and/or system utilizing sensing elements useful in the formation of complex metal piece geometries, such use without excessive tooling modification or set-up time.
It can also be an object of the present invention to provide a sensor device and/or sensing element which can, during the die try-out period, measure and monitor the amount of draw-in, thereby, assisting tooling modification and the determination of process parameters to delay/avoid tearing or wrinkling failure and to keep springback under control.
It can also be an object of the present invention to provide a sensor device and/or sensing element which can provide quantitative measurements of local material movements, to which can be compared with numerical simulations.
It can also be an object of the present invention to provide a sensor device and/or sensing element which can, during a metal-forming process, measure, control and monitor the amount of draw-in material, thereby reducing the incidence of material wrinkling or tearing failure.
It can also be an object of the present invention to provide a sensor device and/or sensing element which produces an output indicative of real-time measurement of the draw-in movement of a metal sheet/plate, such measurement as can be utilized for closed-loop control of a metal-forming process to achieve the optimal (or predetermined) forming condition regardless of variations in incoming material, working environment, operators and process variables. The sensor can be used to provide an indication at the beginning of the forming process to indicate whether the blank is placed correctly at the desired initial position.
It can be another object of the present invention to provide a durable, robust sensing element for use in a variety of metal-forming systems and operations, such as for the measurement of the retreat or movement of the edge of a metal sheet into a mold or toward a deformation zone during a forming operation.
Other objects, features, benefits and advantages of the present invention will be apparent from this summary and the following descriptions of various preferred embodiments, and will be readily apparent to those skilled in the art having knowledge of various metal-forming operations and/or control techniques. Such objects, features, benefits and advantages will be apparent from the above as taken into conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom.
In part, the present invention includes a system for sensing the draw-in amount of metal during a forming process, thereby effecting related elements of monitoring and control. A sensor of this invention can include a sensing element and an electronic circuit, which provides a signal, indicative of draw-in amount, to a computer (or a processor) which, in turn, can control aspects of the forming operation (such as, pressure on the binder at different places, drawbead penetration height, etc.). Benefits of such a sensor system/sensing element include provisions of 1) a tool for process optimization during the development stage; 2) control over the forming process to significantly reduce part-to-part variations and compensate for any incoming material variations; and 3) a device for quick process diagnosis during production.
In one embodiment, the sensing element comprises a pair of coils disposed in signal coupling relationship such that an excitation signal applied to one coil produces a signal in the other coil. The presence of a ferrous or non-ferrous metal, such as a metal plate, in the proximity of the coils affects the magnetic field lines, changing the degree of mutual inductance, or magnetic coupling, between the coils in correspondence with the amount of movement of the metal plate relative to the sensing element. Thus, the response signal, or signal output of the sensing element, provides a real-time measurement of the extent of overlap of the sensing element by the edge portion of the metal plate being formed. The response signal can be supplied to a process controller for process diagnosis and closed-loop control of the forming process.
In part, the present invention can also include methods and related aspects of sensing element fabrication. Fabrication can be in conjunction with and/or on conventional printed circuit board or flexible printed circuit board. Likewise, fabrication of a conductive pattern can be achieved through any of a number of techniques or procedures each of which will be well-known and understood by those skilled in the art and made aware of this invention.
Accordingly, there is provided herein a process for fabricating a sensing element for use in measuring the amount of movement of an edge of a metal plate towards a deformation zone during a forming process. In one embodiment, the fabricating process is carried out using a contact printing and etching technique wherein a conductor pattern is formed on a detachable polymer that is removably applied to a cylindrical surface of a forming cylinder. After contact printing of a masking pattern onto a thin layer of conductive material disposed on the polymer, the conductive material is selectively etched to produce the conductors forming the coils of the sensing element, and the conductor bearing polymer is then cut and peeled off the forming cylinder.
Further in accordance with the invention, there is provided a method for providing real-time measurements of draw-in for closed-loop control of a metal forming process in which a blank of sheet metal, held in place between cooperating tools a die of a sheet metal forming apparatus, is drawn into a deformation zone of the forming apparatus by a further tool. The method includes locating a sensing element on one of the tools, the sensing element including first and second coils disposed in signal coupling relationship; applying an excitation signal to the first coil for inducing a signal in the second coil; and producing a feed-back from the signal induced in the second coil, as the sheet metal is drawn into the deformation zone, for controlling a parameter of the forming process as a function of the amount of retreat of an edge of the sheet metal blank into the deformation zone. For example, the feed-back produced as the result of monitoring draw-in during stroke enables modifying the tooling or changing local press parameters, such as drawbead shape, drawbead height, type, cylinder pressure, etc. Moreover, the measurement results can be used to test accuracy of numerical simulations by comparing the simulation with experimental results and can be used a quality control tool during real-time operating mode. In addition, such measurements provide information that can be used to adjust local or subsequent forming processes to ensure greater parts consistency even for varied material lots. Also, measurements can be continuously plotted throughout the stroke. The actual draw-in can be determined if it is recorded during stroke regardless of springback.